Actually, I want it to be more specific than that. I'd like to have Brand and fuel name included as well as a fuel type identifier - so, BP Ultimate 98 and 98 RON and most entries you would just note the fuel brand/type and the system would have a single store of information on that fuel type.
We're aiming Fuelly for a mainstream audience of drivers trying to improve their economy, so our focus is on keeping things simple and not mucking up our data entry forms with too many options that would lead people to think adding fuel ups is a ton of work mentally.
That said, you can track different fuel types or grades in the notes field about each fuel up and I've seen users say their ethanol % with each fuel up.
Also, I'd consider myself somewhat of a car guy and I honestly have no idea what RON even means, so I suspect the vast majority of our users would be confused by an option for it as well.
Well, that would be obvious wouldn't it? If you filled up say last week with X octane fuel at X station, then the next fuelup would actually reflect the impact on your usage/mpg/etc from that fuelup last week. And if you use the tags to track those changes, you will have a good idea what type of gas from which station is helping you the most. This would also of course depend on whether your driving habits remained constant too though.
I find that with all my driving that my habits may stay the same but the usage level changes, depending on how much city vs country driving I do on that particular tank. I rarely ever let the car idle for more than a minute, if I actually leave the vehicle it gets shut off, I don't warm it up in the winter, just get in and go (just go easy).
Anyways, tags are useful in knowing where you got your last tank from, and what impact (if any) it has on your mileage. Keep in mind, the true impact differences are going to be slight anyway. More octane does not give you more mileage. Octane is a spark knock prohibitive and normally only needed for cars that require higher octane fuel due to their higher compression ratios. So unless you car says to use a higher octane, don't, or if you car has spark knock, go up one grade to try to help it.
How about then using those tags to get comparative fuel economy between different octane rating and Fuel types?
Because of the lag that DTMAce mentioned, I'm not sure that we could auto-calculate differences based on tags. This might be something you could try in Excel after an export, though. Getting into differences in octane is just a bit more niche than we're going for with the site. I think I've seen some hypermiler sites that do track this level of detail so it might be worth looking around at alternatives.